1. Field of the Invention
The present invention relates to an article of manufacture made of a magnesium alloy such as, for example, a wheel or a suspension arm of an automotive vehicle, and also relates to a method of making the same.
2. Description of the Prior Art
Aluminum alloys are generally used for the manufacture of a wheel, a suspension arm, or the like of an automotive vehicle for the purpose of reducing the weight of the automotive vehicle. In recent years, magnesium alloys tend to be used in order to aim at a further reduction in weight. Japanese Laid-open Patent Publication (unexamined) No. 59-67102 discloses a disc wheel which is obtained by casting a molten magnesium alloy under a high pressure to provide a wheel casting and by carrying out a T6 treatment (a solution treatment and an artificial aging treatment) with respect thereto.
In the case of the magnesium alloys, however, the manufacture of the entire wheel only by the casting results in insufficient strength of, for example, a rim portion which carries a tire. Because of this, the problem arises that physical properties of the wheel cannot be sufficiently improved even by the T6 treatment.
On the other hand, the so-called casting and forging is known as a method of making an article of manufacture such as an automotive part or the like, and comprises the steps of: casting a metal alloy to provide a casting (a blank to be forged) of a shape analogous to that of a target product; and forging the casting. For example, in applications where an automotive wheel is manufactured by the casting and forging, a molten alloy 103 is initially poured into a cavity 102 defined in a sand mold 101, as shown in FIG. 30, to provide a forging stock 104 shown in FIG. 31 for finisher-forging. This forging stock 104 is then placed in between and finisher-forged by upper and lower dies 105 and 106, as shown in FIG. 32, thereby providing a forging. Thereafter, the forging is held between a mandrel 107 and a pressure member 108, as shown in FIG. 33, so that only a rim portion may be subjected to spin forging with the use of a roll 109. The finisher forging may be omitted according to circumstances.
As discussed hereinabove, the casting and forging has the advantage of considerably simplifying the required processes as compared with the normal forging in which a billet is used as a stock. Because of this, this method is primarily applied to the manufacture of articles made of an aluminum alloy. This method, however, has little advantages in improving the physical properties of such articles.
More specifically, the aluminum alloys include casting alloys having a high strength, and a specific casting process for increasing the strength such as, for example, liquid metal forging can be applied thereto. Accordingly, it is possible to make high-strength parts of a shape close to that of a target product only by the casting. Furthermore, even if the crystalline particle size of a casting of an aluminum alloy is micronized by forging, remarkable effects cannot be expected in respect of the physical properties.
Japanese Laid-open Patent Publication (unexamined) No. 4-147938 discloses an article of manufacture made of a magnesium alloy and a method of making such an article. According to this disclosure, the so-called sputtering process is applied to the manufacture of a magnesium alloy containing a high-melting metal, for example, tantalum (Ta) or niobium (Nb). By the application of the sputtering process, magnesium together with tantalum or niobium is held as a solid solution of a uniform phase. The magnesium alloy thus obtained is light and superior in resistance to corrosion, resistance to wear, and toughness. However, the problem arises that because each of tantalum and niobium has a high-melting point, the use of the sputtering process is inevitably required, thus complicating the manufacturing method.